Use MustNewConstMetric to collect proxy metrics

[Lusitaniae]

I think that this fix is not correct, we should instead use a MustNewConstMetric and collect it on every scrape.

The current approach is fragile because if 2 prometheus scrape at the same time, they will get hazardous results.

Originally posted by @roidelapluie in https://github.com/Lusitaniae/apache_exporter/issues/111#issuecomment-922882394